The behaviour of nematic liquid crystals in chemically patterned millimetre-sized rectangular surfaces

We describe a method to produce nematic liquid crystal ‘semi-droplets’ with potential use in sensing by chemically patterning rectangles on solid surfaces, surrounded by a hydrophobic monolayer to confine the liquid crystal. The substrate has directional planar anchoring, while the top surface is a liquid crystal/air interface. The effects of varying the semi-droplet of E7 height between ~20 and 420 μm; aspect ratios between 0.5 and 1; widths between 6 and 13 mm and lengths between 6 and 20 mm were investigated, significantly extending the range of semi-droplets considered in the literature; their increased size is expected to be useful in real-life devices. A geometric model for the semi-droplet was derived and compared with an existing theoretical model, offering a simple method to predict the semi-droplet interface angle, which can subsequently be used to predict the maximum height and height variation along the semi-droplet. Polarised light imaging was used to deduce the semi-droplet height profile using the liquid crystal’s birefringent properties, with excellent agreement with the model for heights ≲ 50 μm, even when the semi-droplet width was larger than E7’s capillary length. It is suggested that the confinement surface is reusable by washing with the relatively cheap and safe solvent isopropanol.